Watergel explosives, also known as slurry explosives, are widely used in many applications. They perform well and offer advantages over conventional nitroglycerine-based explosives in terms of improved safety in manufacture, use and storage.
In essence a watergel explosive consists of a fluid mixture of oxidiser salts, fuels, thickeners, sensitisers and crosslinking agents. Normally, watergel explosives are two phase systems (which can be prepared as a single phase), and contain between 10% and 30% water. Thus, a portion of the oxidiser salts and occasionally the sensitiser are dissolved in the water and the balance are dispersed in the solution. To improve the dispersion capacity of the solids in the solution, thickeners that raise the viscosity of the system are added, thus ensuring a greater homogeneity of the final product.
Because of the high water content, the product initially has a fluid consistency that allows easy pouring Of it directly into the blast holes. However, as long as the product remains fluid it is not possible to pack it in the standard paper cartridges used, for example, for packaging nitroglycerine-based explosives as the watergel wets the paper and it disintegrates. The consistency of a watergel thus necessitates cartridging of the product in hermetically sealed plastics containers, generally of high or low density polyethylene. Watergels cartridged in plastic do not load well into boreholes because the plastic packaging resists breaking thereby preventing the explosive from filling the boreholes properly. Also, the plastics packaging itself is not suitable for use in many mines, for example asbestos mines and diamond mines.
The gelatinous and powder explosives sensitised with nitroglycerine, nitroglycol or other nitrates of alcohols or mixtures thereof, have the advantage that they can be easily cartridged in paper. However, the use of these explosives presents problems for the user, on the one hand because these sensitisers are vasodilators and they cause headaches and other circulatory problems, and on the other hand because the detonation fumes contain a high percentage of toxic gasses such as oxides of nitrogen and carbon monoxide.
In European patent EP 0187709 an attempt is made to overcome the problems associated with the packaging of watergel compositions by providing a paper-based cartridge that has at least one of its surfaces coated with a resin, which is resistant to water-based explosives. The patent describes the production of a waterproof paper cartridge, which is simply made of plastic-faced paper and it is made on a special machine constructed for this purpose. The explosive packaged in the cartridge is, however, a standard watergel or emulsion explosive. The resistance of the package to wetting by the watergel is entirely dependent on the integrity of the plastic layer in contact with the water gel within the package. In the patents ES 2005367, U.S. Pat. No. 4420440 and U.S. Pat. No. 4756776 procedures or methods, machines or apparatus are claimed for cartridging explosive formulations without claiming the specific products to be made and to be cartridged according to the procedures described in them.
To give watergels a better consistency and water resistance, thereby avoiding the leaching of the salts by external water and a resultant loss of explosive performance, the gel is crosslinked. In this form the product takes on an elastic consistency after a certain time that is impossible to shape and manipulate for cartridging, which is why the packaging is done while the material is still fluid. The crosslinked product, however, still wets the paper, making this an unsuitable packaging material for watergels of the prior art.
Generally, in explosive compositions sensitised with monomethylamine nitrate or other alkylamine or alkanolamine nitrates, the quantity of sensitiser is much greater than 15%, and frequently greater than 20%. In U.S. Pat. No. 4,096,003 a method is proposed to produce a watergel using only 8% monomethylamine or monoethanolamine nitrate as well as utilising a well known sensitiser for watergels, namely pigment aluminium, as a supplementary sensitiser. (Pigment aluminium is finally divided aluminium, often in the form of flat flakes, which is used as a sensitiser in watergel compositions. It typically has a surface area in excess of 5000 cm.sup.2/ g.) However, the watergels described in U.S. Pat. No. 4,096,003 all contain greater than 10%, by mass, of water.